C++ I/O library (experimental)

This is an experimental snapshot of SGI's standard C++
library. You should download it if you are
interested in contributing to an ongoing development effort, but not
(yet) if you are looking for a library implementation that is
guaranteed to work "out of the box".

This snapshot includes the entire standard C++ library: STL,
iostream/locale, and numerics (valarray and
complex). It does not include a standard C library
implementation; it is intended to interoperate with an existing C
library.

Please note that some aspects of an I/O library are inherently
platform-specific. (See below for more details.) This library has
been tested on IRIX 6.x using the SGI MIPSpro 7.3 compiler, and on
Linux, using glibc 2.0 and the egcs 1.1.2 compilers. It passes simple
tests on Microsoft Windows NT using the Microsoft Visual C++ 6.0
compiler (with Service Pack 3). Compiling it on other platforms will
almost certainly require at least some work. Porting to another
version of Unix, using a relatively recent C++ compiler, should not be
difficult.

We welcome suggestions, patches, and bug fixes.

Some notes on the structure of the I/O library

It shares some files, such as char_traits.h, with our STL. These
snapshots include the full STL distribution as well as the
iostreams implementation, since it's hard to draw a line between
these two parts of the standard C++ library.

Certain parts of the STL depend on the I/O library, and they
behave differently depending on whether they use "classic"
AT&T cfront iostreams or a standard-conforming I/O
implementation with templatized iostreams. To use the new iostreams
library you must tell the compiler where to find the headers,
add the I/O library to the link line, and tell the STL that it should
use standard-conforming iostreams. A compilation line may look something
like this:

g++ -I. -D__STL_USE_NEW_IOSTREAMS -g hello.cxx libCio.a

Alternatively, you can edit the stl_config.h header so
that the macro __STL_USE_NEW_IOSTREAMS gets defined
automatically.

Locales are a tricky point. It is impossible to layer C++ locales
on top of the C library locales. The problem is that the standard
C library only has one active locale at a time, as determined by
the global setlocale function. In C++, though, locales
are objects, and there can be any number of active locales at the
same time. In a single-threaded world it would be possible
(although horribly inefficient) to implement this with
setlocale, but there's no safe way to do that in a
multithreaded environment.

This iostream/locale library is intended to coexist with a
preexisting C library. It assumes that for any C library
implementation there is some way---just not a portable way---to
get at the underlying data files so that we can have multiple
active locales. We defined an interface, found in in
c_locale.h, for this functionality. Our C++ I/O library
requires some implementation for the c_locale.h
interface. We believe that implementing the c_locale.h
interface is the major work involved in porting the I/O library to
a new platform.

We have implemented the c_locale interface for the IRIX C library
(it is not included here because it includes code that, for legal
reasons, SGI can't release as open source), and for the GNU glibc
2.x library.

Additionally, we provide a stub implementation of the c_locale
interface. This interface is used only for named locales that the
OS knows about, such as (in IRIX) "de",
"es_MX", and "fr_BE". It isn't used for
user-defined locales, or for the classic locale. So the stub
implementation takes any string, and returns a failure indication
when the user attempts to create a named locale. That's perfectly
conforming, according to the C++ standard, and it's even useful.
(At least for people who don't have a pressing need for
internationalization.) This can serve as a stopgap for platforms
where we don't yet have a real c_locale implementation.

There are a couple of smaller places where we have
system-dependent code. One, of course, is in
basic_filebuf (see especially fstream.cxx),
which makes system calls for reading and writing. (On Unix, it
uses such system calls as open, close,
read, write, fcntl, lseek,
mmap, and munmap.)

We have tried to abstract out as much system-dependent stuff as
possible into the _Filebuf_base class. At present it
supports IRIX and Linux; porting to other versions of Unix should
be easy. There is also some incomplete support for NT; issues
there include the '\n' <-> CRLF translation in text mode,
and the use of native Win32 system calls, especially for memory
mapped I/O.

Another place where we've got system dependent code is syncing
with stdio. The C++ standard requires that there be a mode in
which iostreams and stdio are synchronized (the C++ standard is
imprecise about just what synchronization means, but, after
talking to the people who wrote that part of the standard, it's
clear that it means a very close coupling), and it requires that
synchronized mode be the default. We could have satisfied this
requirement by implementing things in terms of putc and
getc, but that would have been horribly inefficient.
Instead we did something uglier, but more efficient: we directly
manipulate the pointers inside a FILE object. We've
encapsulated knowledge of struct FILE into the header
<stl_stdio_file.h>, which defines a few inline functions
for this manipulation. Again, this header has to be modified for
every OS that we target. This has not proven to be a serious problem
in practice, since most stdio implementations appear to be very
similar to each other.

(An aside: the C++ standard does not say what type of streambuf is
used by the standard streams, cin, cout,
cerr, etc. Our choice: the wide streams, and in
unsynchronized mode the narrow streams as well, use
basic_filebuf. In synchronized mode the narrow streams
instead use SGI::stdio_istreambuf or
SGI::stdio_ostreambuf. Those classes are defined in the
header <stdio_streambuf>, and, of course, they are extensions.)

Initialization of iostreams is tricky. The standard specifies how
it's supposed to work in some detail, and it doesn't seem to be
possible to do it portably. Our solution, which is admittedly
a ghastly hack, is in iostream.cxx. It's nonportable, but
it will work on many platforms. On some platforms there are better
ways of doing this.

Areas that need more work

Portability to other platforms

Workarounds for compilers that don't support the full C++ language

Writing a low-level locale implementation for other C libraries.

Use native file I/O system calls for non-Unix operating systems.

Testing on other configurations

The basic_fstream class sometimes uses memory-mapped I/O
for file input. It should also use memory-mapped I/O for output
when possible.

Testing of nontrivial code conversions from wide to narrow characters,
especially with variable-width and state-dependent encodings, has
been sporadic; it's likely that there are still bugs in this area.
Most of the relevant code is in <fstream>.

The numeric input facets handle istreambuf_iterator
naively. They should look directly at the buffer inside the
streambuf, rather than using sgetc, sbumpc,
and snextc to get one character at a time. (See the
header <istream> for an example of how this can be done.)
The same optimization applies to the time and monetary facets, but
they're less important.

There has been some performance tuning for better execution speed,
but there has been very little performance tuning for code size
or compilation speed.

Many small bug fixes and compatibility patches. Change to the
low-level locale API to improve handling of locale "", which
indicates that we are to use a locale that corresponds to the user's
preference. (The major changes are in c_locale.h and locale.cxx.)
This change is probably a performance improvement in the common case.

Changed basic_istream::seekg and basic_ostream::seekp
so that they conform to the resolution to library issue 136
adopted in Tokyo. (seekg should set only the get pointer,
and seekp should set only get put pointer.) Fixed a bug that
affects string output when the width is nonzero; thanks to Volker
H. Simonis for a bug report and patch. Changed bitset to
remove the extension of a second template parameter. The extension
does not appear to be standard-conforming (see library issue 94),
and it isn't very useful anyway.

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